The present invention relates to a color image forming apparatus for forming a color image on a fluorescent screen or a photosensitive recording medium which has plural sensitivities to lights of plural wavelength ranges, and more particularly to a color image forming apparatus having an image intensifier unit for intensifying an image light to thereby form a color image having a high output power with high quality and high resolution.
In recent years, there have been known various color image forming apparatuses for forming a color image on a color-image reproducible photosensitive recording medium. With the advance of digitization of image formation, an apparatus utilizing a semiconductor laser or a light-emitting diode (LED) as a light source for producing an image light has been particularly put to practical use.
At present, semiconductor lasers can be practically used in only specific wavelength ranges, for example, from 780 to 830 nm (for CD players, laser printers, and like), or 1550 nm (for general communication systems). Further, LEDs can be practically used in only specific wavelength range of 650 nm, 750 nm or 840 nm, and hence the LEDS as well as the semiconductor lasers has severe restriction to a practically usable wavelength range. If a color image is recorded on a photosensitive recording medium using an image light which has a wavelength located in the wavelength ranges as described above, then it is necessary to develop recording medium materials having sensitivities which match with those wavelength ranges. However, some photosensitive recording media such as silver-salt photographic films and photosensitive microcapsule-coated sheets (see Japanese Unexamined Published Patent Application 58-88739 and U.S. Pat. No. 4,399,209, for example) have sensitivities which do not match with the wavelength ranges of the semiconductor lasers or the like. For example, since the spectral sensitivity of the silver-salt photographic films is in a short wavelength range of from 400 to 700 nm, no semiconductor lasers can be used for the silver-salt photographic films. Further, there is a special silver-salt photographic film having a sensitivity to light in the infrared range (for example a photographic film manufactured by Kodak). However, the cost of energy required by this silver-salt photographic film is higher since its sensitivity to light in the infrared range is lower than that in a shorter wavelength range and a higher intensity of light is needed to perform an exposure process. Similarly in the silver-salt photographic film, the semiconductor lasers can not be used for the photosensitive microcapsule-coated sheets as described above since the spectral sensitivity of the sheets is also in a short wavelength range of from 400 to 700 nm. In view of the foregoing, there has been developed semiconductor lasers which could be used for a photosensitive recording medium having a spectral sensitivity to light in such inherent wavelength range. However, the cost of energy required by such semiconductor lasers is higher because of the short wavelength range.
On the other hand, there has been also known an image forming apparatus in which a color image is recorded using as a light source for producing an image light an image display such as a cathode-ray tube (CRT), a liquid-crystal display (LCD), a light-emitting diode (LED) display, an electroluminescence (EL) display, a plasma display or the like. However, these image displays have the following disadvantages, and therefore an image forming apparatus utilizing one of these image displays has not been hitherto put to practical use.
A CRT has a luminescent screen coated with a fluorescent material for emitting on the basis of an image information an image light whose wavelength matches with the spectral sensitivity of a photosensitive recording medium. With such an arrangement, the wavelength range of the image light and the photosensitive wavelength distribution of the photosensitive recording medium can be easily matched with each other. However, in this apparatus, if an output power of the electron beam in the CRT is increased, the diameter of a beam spot cannot be reduced. As a result, no higher brightness can be achieved b/ this apparatus and the edges of recorded images are blurred. Further, the depth of the CRT itself must be increased as the CRT is designed to be larger in size, so that the volume of the CRT is increased by the third power of the length. Still further, since the internal space of the CRT is kept under vacuum condition, a thickness of a glass housing of the CRT must be increased to prevent the glass housing from exploding due to the vacuum condition. Consequently, the weight of the CRT is also increased.
Further, the LCD has a higher production cost if a TFT (thin film transistor) is employed for solving the problem of visual angle. In addition, the cost is also increased if a redundancy circuit is provided for providing a large scale display.
Still further, in case of an image display using a semiconductor technique such as LED and EL, since a light emitting efficiency at short wavelength is low, this image display is incapable of practical use. The plasma display suffers a lower light emission brightness due to degradation of the discharge gas.